Solar Energy Storage by Molecular Norbornadiene–Quadricyclane Photoswitches: Polymer Film Devices

Abstract

Devices that can capture and convert sunlight into stored chemical energy are attractive candidates for future energy technologies. A general challenge is to combine efficient solar energy capture with high energy densities and energy storage time into a processable composite for device application. Here, norbornadiene (NBD)–quadricyclane (QC) molecular photoswitches are embedded into polymer matrices, with possible applications in energy storing coatings. The NBD–QC photoswitches that are capable of absorbing sunlight with estimated solar energy storage efficiencies of up to 3.8% combined with attractive energy storage densities of up to 0.48 MJ kg −1 . The combination of donor and acceptor units leads to an improved solar spectrum match with an onset of absorption of up to 529 nm and a lifetime ( t 1/2 ) of up to 10 months. The NBD–QC systems with properties matched to a daily energy storage cycle are further investigated in the solid state by embedding the molecules into a series of polymer matrices revealing that polystyrene is the preferred choice of matrix. These polymer devices, which can absorb sunlight and over a daily cycle release the energy as heat, are investigated for their cyclability, showing multicycle reusability with limited degradation that might allow them to be applied as window laminates.

Engineering of Norbornadiene/Quadricyclane Photoswitches for Molecular Solar Thermal Energy Storage Applications

“…Table 1 shows examples of different NBD systems 1 – 8 ( Figure 3 ); it is possible to see the structural evolution of the molecules ( Figure 3 , NBDs 3 – 8 ) with a significant improvement of parameters like absorption of light, quantum yield, energy density, and half-life time compared with the standard NBD 1 and structures proposed by Yoshida et al (NBD 2 ). 2 , 4 , 13 , 15 , 16 , 25 , 26 , 39 , 40 …”

“…The systems with a strong acceptor trifluoroacetyl group presented more red-shifted absorption ( A onset up to 529 nm) compared with the cyanide analogues ( A onset up to 400 nm), allowing a better overlapping with the solar spectrum ( Figure 9 a). 4 The half-lives ( t 1/2 between 0.64 h to 3 days) of NBDs with trifluoroacetyl group are more suitable for applications that require daily cycles compared to their cyano variants ( t 1/2 between 7 h to 55 days). 4 Another derivative with trifluoroacetyl acceptor group, NBD 11 in Figure 8 a, was loaded in polystyrene at different concentrations, and the absorption spectra and cyclability tests showed a decomposition per cycle of 0.02–0.45% ( Figure 9 b).…”

“… 4 The half-lives ( t 1/2 between 0.64 h to 3 days) of NBDs with trifluoroacetyl group are more suitable for applications that require daily cycles compared to their cyano variants ( t 1/2 between 7 h to 55 days). 4 Another derivative with trifluoroacetyl acceptor group, NBD 11 in Figure 8 a, was loaded in polystyrene at different concentrations, and the absorption spectra and cyclability tests showed a decomposition per cycle of 0.02–0.45% ( Figure 9 b). However, studies in more concentrate composites were not carried out due to the fast back conversion rate that makes it impossible to have a full conversion of these NBDs in the composites.…”

“…The heat released by the NBD/polystyrene composites (0.8 wt %) was measured in the range of 2.7–3.0 kJ/kg. 4 …”

Accurate prediction of norbornadiene cycle enthalpies by DLPNO‐CCSD(T1)/CBS method

“…[1] Practical interest in norbornadiene thermochemistry is based on the potential use of the norbornadiene-quadricyclane isomerization as a solar energy storage system. [2][3][4][5] Although the enthalpies of formation (Δ f H ) of these compounds have been measured repeatedly (see References [1,[6][7][8][9] and references therein), there is still disagreement in their values. Only the experimental value of Δ f H (g) for norbornane recommended by…”